A developing system is known for an electrographic system or an electrostatographic system of a type in which a mixture of a magnetic carrier and a toner (magnetic or non-magnetic), or a developer comprising a magnetic toner is used. A magnetic brush is formed on a hollow cylindrical rotatable sleeve made of a non-magnetic material and incorporating a permanent-magnet member. The magnetic brush rubs the surface of an image carrier to make an electrostatic charged image visible.
FIGS. 1 and 2 are a cross-sectional view and a side view illustrating a developing system of a conventional type. In FIGS. 1 and 2, numeral 1 refers to a developer tank housing a developer formed by mixing a magnetic carrier with a triboelectric type magnetic toner (see Specifications of U.S. Pat. Nos. 4,640,880 and 5,064,739) and having on top thereof a toner cartridge housing hole 2. Numeral 3 refers to a movable member or paddle means for transferring and mixing the developer and is rotatably provided in the developer tank 1. Numeral 4 refers to a sleeve made of a non-magnetic material, such as aluminum alloy, stainless steel, etc., formed into a hollow cylindrical shape, and rotatably fitted in the developer tank 1.
Numeral 5 refers to a permanent-magnet member formed into a cylindrical shape by fixedly fitting a cylindrical ferrite magnet to a shaft. The outer circumferential surface of the permanent-magnet 5 has a plurality of magnetic poles extending in the axial direction, and is fitted to the developer tank 1 concentrically and relatively rotatably with respect to the sleeve 4. Numeral 6 refers to a doctor blade provided above the sleeve 4 at a predetermined gap from with the sleeve 4. Numeral 7 refers to guide rollers having a diameter slightly larger than that of the sleeve 4 and provided at both ends of the sleeve 4 in the longitudinal direction in such a manner as to come in contact with the surface of an image carrier 8.
Numeral 9 refers to a supporting shaft supported by supporting frames 10 provided outside of the developer tank 1 in the longitudinal direction in such a manner that the developer tank 1 can be rotated and the guide rollers 7 can be engaged with and disengaged from the surface of the image carrier 8. Numeral 11 refers to a cam formed in such a manner as to be rotatable by a lever 12, and slidably engaged with the cam surface 13 formed on the bottom of the developer tank 1. Numeral 14 refers to a tension coil spring interposed between the developer tank 1 and one of the supporting frames 10.
With the above construction, as the lever 12 is thrown to a position shown in FIG. 2, a clockwise rotating force is imparted to the developer tank 1 by the tension of the tension coil spring 14, causing the guide rollers 7 to come in contact with the surface of the image carrier 8. This allows the gap between the sleeve 4 and the surface of the image carrier 8 to be kept constant.
In this state, as the sleeve 4 is caused to rotate counter-clockwise, the permanent-magnet member 5 and the movable member or paddle means 3 are caused to rotate clockwise. The magnetic carrier and the magnetic toner are agitated uniformly in the developer tank 1 by the movable member or paddle 3, and electrostatically charged by friction, attracted onto the surface of the sleeve 4 by the magnetic attraction of the permanent-magnet member 5, and transferred to a developing area to form a magnetic brush. Thus, the magnetic brush rubs the electrostatic charged image on the surface of the image carrier to make the image visible, that is, to develop the image.
Upon completion of developing operation, or during maintenance inspection, as the lever 12 is turned 90 degrees clockwise, the cam 11 is moved while sliding on the cam surface 13, causing the developer tank 1 to turn 6-10degrees, for example, counterclockwise around the supporting shaft 9. This causes the guide rollers 7 to be disengaged from the surface of the image carrier 8. Since the entire system can be extracted in this state in the direction vertical to the paper surface as necessary, component members can be subjected to maintenance, inspection and repair.
With the developing system having the aforementioned construction, and since the guide rollers 7 are rotated while coming in contact with the surface of the image carrier 8, the gap between the sleeve 4 and the surface of the image carrier 8, that is, the developing gap, can be kept constant. There remains a problem, however, that the developing gap cannot be maintained at a predetermined value over the overall length of the sleeve 4 because the entire system may be distorted or deformed by errors caused during assembly of the developing system and other factors.
The aforementioned developing gap is generally maintained at less than 1 mm, and often within the range of 0.2-0.6 mm when a developer containing a magnetic toner, or a two-component developer using a ferrite carrier is used.
The aforementioned problem is attributable to the fact that the connecting parts of the supporting shaft 9 and the supporting frames 10 are tightly constructed at both ends of the supporting shaft 9, leaving less room, or less play, in engagement. Even when the manufacturing accuracy of the component members, on the other hand, is improved, and the assembly accuracy among the component members are strictly controlled, it is almost impossible to perfectly bring both the guide rollers 7 and the image carrier 8 into contact with each other without errors. Such efforts would prove uneconomical because of an increase in manufacturing cost.
To overcome the aforementioned problems, Japanese Published Examined Utility Model Application No. 33249/1990, for example, proposed a construction that a developing system is disposed on an electrostatographic system by supporting a positioning shaft for the developing system in an almost horizontal position in developing-system supporting grooves provided on the electrostatographic system; one of the supporting grooves being formed into almost the same size as the outside diameter of the positioning shaft, and the other supporting groove being formed into such a size as to allow a slight play with the positioning shaft. This construction, however, can absorb only a localized load generated by the distortion of the developing system itself by the aforementioned play, and is short of reliability because it does not positively push the developing system itself onto the image carrier.
In recent years, on the other hand, the functional and economical requirements for developing systems of this type have been increasingly stringent to such an extent that the conventional systems cannot satisfy such needs.